Curly-Braces

Curly-braces should be omitted in cases where the control structure
represents a pure-functional operation and all branches of the control
structure (relevant to if/else) are single-line expressions.
Remember the following guidelines:

if - Omit braces if you have an else clause. Otherwise, surround
the contents with curly braces even if the contents are only a
single line.

while - Never omit braces (while cannot be used in a
pure-functional manner).

for - Omit braces if you have a yield clause. Otherwise,
surround the contents with curly-braces, even if the contents are
only a single line.

While the latter style is more verbose, it is generally considered
easier to read and more “scalable” (meaning that it does not become
obfuscated as the complexity of the comprehension increases). You should
prefer this form for all for-comprehensions of more than one
generator. Comprehensions with only a single generator (e.g.
for (i <- 0 to 10) yield i) should use the first form (parentheses
rather than curly braces).

The exceptions to this rule are for-comprehensions which lack a
yield clause. In such cases, the construct is actually a loop rather
than a functional comprehension and it is usually more readable to
string the generators together between parentheses rather than using the
syntactically-confusing } { construct:

Finally, for comprehensions are preferred to chained calls to map,
flatMap, and filter, as this can get difficult to read (this is one
of the purposes of the enhanced for comprehension).

Trivial Conditionals

There are certain situations where it is useful to create a short
if/else expression for nested use within a larger expression. In
Java, this sort of case would traditionally be handled by the ternary
operator (?/:), a syntactic device which Scala lacks. In these
situations (and really any time you have a extremely brief if/else
expression) it is permissible to place the “then” and “else” branches on
the same line as the if and else keywords:

val res = if (foo) bar else baz

The key here is that readability is not hindered by moving both branches
inline with the if/else. Note that this style should never be used
with imperative if expressions nor should curly braces be employed.